"They don't rest when there are tour boats near them," says Harcourt, a marine behavioural ecologist. "It might be a long term stressor."

Port Stephens in New South Wales is a national marine park that is home to a small population of Indo-Pacific bottlenose dolphins (Tursiops aduncus), and to Australia's largest dolphin watching industry.

In 2008, the port had more than 270,000 visitors and over 80 per cent of them came to marvel at the dolphins.

Harcourt and colleagues investigated the impact of commercial dolphin-watching boats from August 2008 to August 2009.

Dolphins 'sleep' by taking short floating breaks regularly through the day and night, resting half of their brain at a time.

But the researchers found that the dolphins in Port Stephens stopped taking rests when the boats came within 100 metres of them.

When the boats came within 50 metres, the animals spent 66.5 per cent less time feeding and 44.2 per cent less time socialising and four times more milling than normal, they report.

And the researchers found boats approached groups of dolphins with newborns closer than the minimum state regulated distances 9 out of 10 times.

Industry response

According to recent marine park regulations, boats are not supposed to approach within 150 metres of a dolphin pod with calves, but the commercial dolphin watching industry says this is too far.

"More than half of the dolphins that we watch in the port are mothers with calves," says Frank Future from Whale and Dolphin Watch Australia Inc.

"You wouldn't be able to see them at all at 150 metres. That's one and half lengths of a football field."

Under the industry's own code of conduct, the commercial boat operators have been restricting approaches to 50 metres from dolphins since 2006, and they want to continue to do this, regardless of whether there are calves.

"The port sits on the backs of the dolphins to a huge extent," says Future.

"Obviously we have concerns that anything we might do would interrupt the animals but I guess everything is always a compromise."

He says the industry has to consider losing business to states like Queensland and Western Australia where people can get right up close and feed dolphins.

Future says the industry has been given verbal authority from the regional director of the NSW National Parks & Wildlife Service Rob Quirk to continue approaching within 50 metres of dolphins with calves.

Comment from the parks agency was not forthcoming but Future says in return for the 50 metre approach distance the industry has agreed to tighten other controls.

For example, they say they will reduce the amount of time dolphins can be watched, the number of boats that can watch them at one time and the areas of the bay the boats cover.

Refuges?

But as always the devil is in the detail. For example, while industry says they will not go into the western half of the bay, the researchers say this is not necessarily helpful.

Harcourt's colleague Dr Andre Steckenreuter, who carried out the Port Stephens research for his PhD says most of the dolphins are adapted to living in the eastern half of the bay.

He says it's not clear they would be able to use the western half of the bay as a refuge if needed.

Steckenreuter says as part of the upcoming review of the marine park plan he will suggest there should be an exclusion zone in the eastern half of the bay.

Dolphins also have to deal with recreational vessels from the several hundred or so people who spend their summer holidays on the bay.

"When the jet skis come here in summer, they tear around all over the top of them," says Future.

Steckenreuter says he has seen up to 15 boats watching dolphins at any one time, and many of the regulations that apply to commercial vessels don't apply to recreational vessels.

The researchers say the effects are likely to be similar on other small resident groups of coastal dolphins that are the target of dolphin watching industries around the world.